Method of improving the photoconducting characteristics of layers of photoconductive material



July 2, 1968 A. s. ESBITT ET I 39 l 3, METHOD OF IMPROVING THE PHOTOCONDUC G CHARACTERISTIC 7 OF LAYERS OF PHOTOCONDUCTIVE MATERIAL Filed July 21, 1964 6/7 5 0072i, 4 G45 INLET AVVE'NTOfS 4 IV 5- 558177 ez. w. 6652.

LIAM M. A flUFMA/V ATTORNEYS uflitedswes ABSTRACT OF THE DISCLOSURE I l x The photoconductive characteristics of apreviousl'y constituted layer of photoconducting material are 'improved by heating the layer in a chamber which'jalso contains, spaced from the layer, a mass of the same material of which the layer is formed.

. 3,391,021 Patented July 2; 1968,

ice

. 2' i the voperating characteristics of vapor-deposited photoconducting layers are comparatively poor.

The process of the present invention, which is particularly adapted for use with vapor deposited photoconduc: tive layers but which is not limited thereto, is designed to greatly improve the operating characteristics of preformed photoconductive units. In particular the response time is greatly reduced, dark conductivity is greatly reduced, light conductivity is greatly increased, and the light-to-dark conductivity ratio isvery markedly increased.

In accordance with the present invention the preformed photoconducting unit is placed within a chamber, together in}; the photoconducting characteristics of a previously formed photoconductive layer.

I Conventional photoconductive cells are constructed of a layer of photoconductivematerial upon one surface of which a pattern of electrodes is deposited. The materials in question have the characteristic that their conductivity varies with the amount of light impinging thereon. It is desired that the material have aslow a conductivity as is possible when no light reaches the'mater-ial and that it have as high a conductivity as is possible when'a given amount of light impinges upon the material, thereby to produce as high a ratio as possible of light conductivity compared with "dark'conductivity. It is'further' desirable in many applications that'the'time' lag between a change in light impinging 'onthe material and the corresponding change in conductivity be as short as possiblei It is the prime object of the present inventionto improve these characteristics and thereby produce -a superior photoconductive cell. s i

There are a variety of known photoconductive materials, usually compounds of an elementfrom Group II and an element from Group VI of'the Periodic Table. Cadmium sulphide and cadmium selenide are two of the most commonly used compounds of this character, while cadmium telluride, zinc selenide and zinc tellu'ride are also of significance. It is known that certain doping elements, such as copper, oxygen arid chlorine or other halogens, when added to such photoconductivematerials, improve their photoconducting characteristics various respects. Various methods have been used for the making of such photocells. Usually they are fabricated by sintering powders of these compounds. It has also been proposed to manufacture such substances in theform of films by vapor-deposition, or to grow small single crystals of the appropriate materials. All of these procedures have produced useful devices, but the photoconductive characteristics of such devices have not been: satisfactory. In particular, the ratio of light conductivity to dark conductivity has been relatively low, and the time of response has been relatively high, even though measured in milliseconds. Single crystal photocells have comparatively good performance characteristics, but the high cost of manufacture involved in the'growing of single crystals, and then in the selection of those of theindividual tiny units which exhibit acceptable performance characteris tics, makes such units quite expensivei The main. advantage of the use of vapor-depositedfilms is the cap ability of fabricating large areas of uniform qualitjy, which can then be subdivided into a large number of, units, However,

with a mass of powdered photoconductive material, to which suitable powdered dopants-may be added when de-' sired, the powdered material being spaced from and out of contact with the exposed photoconductive unit. The chamber is then heated to a temperature within a predetermined range in the presence of an inert atmosphere which might also contain appropriate dopant material such as halogens and oxygen. The temperature conditions are such that the thickness of the sample is not significantly increased, the primary result being a marked improvement, by several orders of magnitude, in the significant operating characteristics of the photoconductive unit. It is believed that the physical and chemical changes which take place are quite complex, involving simultaneous doping, dissociation, evaporation, deposition, epitaxial growth and recrystallization, but this explanation is submitted by way of hypothesis, and the invention may well be considered as an empirical one. The original material appears neither to grow nor to evaporate to any appreciable degree, although probably both growth and evaporation take place. Whatever the explanation may be, marked improvement in photoconductive characteristics results. For example, a photoconductive unit having a light:dark conductivity ratio of 1.1 before being subjected to the treatment of the instant invention has that ratio increased to 2x10 after treatment.

To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to a treatment method for preformed photoconductive unit as defined in the appended claims and as described in this specification, taken together with the accompanying drawing showing schematically a typical installation for the practice of the present invention.

Because the present invention appears to be particularly effective in improving the operating characteristics of a vapor-deposited layer of photoconductive material it is here specifically illustrated and described in. connection therewith, although-it will be understood that the inven-' tion is not limited to that particular application. Typically, a layer 2 of appropriate photoconductive material such as cadmium sulphide is deposited on a substrate 4 such as glass in accordance with known techniques, the substrate 4 preferably having been degassed before the vapor deposition takes place. The substrate 4 and photoconductive v layer 2 may occupy an appreciable areal extent (e.g., 20

- conductive layer 2. The charge 8 is spaced from, and does not make contact with, the photoconductive layer 2. Typically the charge 8 is poured into the chamber 6 so as to rest on the bottom wall thereof and only partially fill the chamber 6, the substrate 4 being placed upon the upper surface of the charge 8, the photoconductivelayer 2 being 3 spaced from the charge 8 at least by the thickness of the substrate 4.

The chamber 6 is filled with an atmosphere which is inert, except for such doping ingredients as it may be caused to contain, and preferably the atmosphere is caused to flow through the chamber 6 from an inlet to an out let 12, thereby flowing over the substrate 4. Nitrogen is preferred for the atmosphere, but argon or any other typical inert gas could be employed. The atmosphere also preferably contains a halogen, such as chlorine, since the doping of the photoconductive layer 2 with a halogen is known to improve its photoconductive characteristics, and particularly to produce a high light conductivity and a low dark conductivity. Oxygen may also be included in the atmosphere for similar doping reasons, oxygen increasing the speed of response of the photoelectric unit while somewhat adversely affecting the light-dark conductivity ratio. The precise proportions of halogen and oxygen in the atmosphere are not critical and may be widely varied, depending upon the particular photoconductive characteristics desired in the end product. The halogen may be present as a free element (e.g., gaseous chlorine) or in chemical combination with other elements (e.g., hydrogen chloride). While the manner in which the halogen, and the oxygen if employed, is introduced into the atmosphere is not critical, and many methods are known by which this may be accomplished, it may be here stated, purely by way of example, that nitrogen may be bubbled through a 37% concentrated hydrochloric solution before it enters the inlet 10, a bubbling rate of 1-10 bubbles per second producing an appropriate halogen concentration in the nitrogen atmosphere. When oxygen is to be added, it may be done within the range of .110% by volume.

The chamber, and particularly the substrate 2 and the comminuted charge 8, as well as the atmosphere within the chamber, must be heated, the heater being schematically shown in FIG. 1 and indicated by the reference numeral 14. The temperature will vary depending upon the material of which the photoconductive layer 2 is formed and depending upon the photoconductive characteristics desired. However, in a given application the temperature must be kept within a predetermined range. The temperature should be at least 450 C. and no greater than 800 C., with a temperature range of 550-620 C. being preferred, particularly when cadmium sulphide is involved. If the temperature is too low no appreciable improvement in photoconductive characteristics results. If the temperature is too high appreciable growth of the photoconductive layer 2 results, with no appreciable degree of improvement in the photoconductive characteristics. When, however, the temperature is maintained within the desired range, no appreciable growth of the photoconductive layer 2 results, yet the photoconductive characteristics improve by many orders of magnitude. For a given material the temperature of treatment, in accordance with the present invention, appears to be approximately 40% of the melting point of that material, although the melting of the material does not play any direct part in the present process so far as is known.

It is believed that what takes place within the chamber 6 constitutes a complex balancing of a number of different types of reactions of the cadmium sulphide layer 2 with the gas atmosphere and the powder charge 8. Depending on the relative concentrations of the reacting materials and on the temperature, varying amounts of cadmium chloride, hydrogen sulphide, metallic cadmium, etc. are formed. Recrystallization of the layer occurs which is enhanced by the material transport of the various reactants. Additionally, due to the heating of the charge of powdered cadmium sulphide, an appreciable amount of cadmium sulphide vapor is formed which blankets the layer and prevents excessive evaporation. At the same time the layers become doped with halogen atoms (and oxygen atoms when present).

Doping of the photoconductive layer 2 with additional components can readily be accomplished in accordance with the present invention. For example, if copper doping is desired, a layer of copper powder can be added to the charge '8, and when that is done the light-dark conductivity ratio of the layer 2 is increased, while the response time is somewhat decreased. Other known doping materials could be correspondingly employed to produce their characteristic results.

The length of time during which the process is carried out may be varied widely. Subjection of the photoconductive layer 2 to the treatment conditions for a period of fifteen minutes gives appreciable results, while the treatment may also be performed for periods up to two hours without any degradation of the results.

The various parameters of the process are to some extent interrelated. As the halogen content of the atmosphere is increased, the operating temperature may be decreased. When powdered copper is added to the charge 8, a corresponding increase in the halogen content is usually desired. When oxygen is employed, the temperature or the halogen content must be reduced. Increases in oxygen and copper appear to reduce the time of response of the photoconductive unit while reducing its light conductivity, and an increase in halogen increases the light conductivity.

Improved results are noted when a temperature gradient exists between the powder and the layers or between the two ends of the chamber, which temperature gradient may amount to 300 C.

The halogen content of the atmosphere could be produced by adding an appropriate powdered chloride compound to the charge 8, instead of or in addition to bubbling the inert carrier gas through hydrochloric gas.

As indicative of the surprisingly great improvement in functional characteristics attendant upon the use of the present invention, the following data will be of interest. A vapor-deposited cadmium sulphide pjhotoconductive layer, before treatment in accordance with the present invention, had a dark conductivity of 9.0 10 (in ohms cm. and a conductivity when illuminated with 730 foot candles of 1.0x l0- giving a light-dark conductivity ratio of 1.1. After treatment its dark conductivity was 9.0x 10* and its light conductivity was l.8 10- thus producing a light-dark conductivity ratio of 2x10 The unit, after treatment, had a rise time (the time required for the photocurrent to reach 63% of its steady-state value after the application of 75 foot-candles of light) of 6 milliseconds, and a decay time (defined comparably to rise time) of 0.7 millisecond. Improvements in operating characteristics amounting to several orders of magnitude are consistently attained.

It will be appreciated that many variations may be made in the specific details of the instant procedure, all without departing from the spirit of the present invention as defined in the following claims.

We claim:

1. The method of improving the photoconductive characteristics of a previously formed photoconductive layer, said photoconductive layer comprising a compound of elements from Groups II and VI respectively of the Periodic Table, which layer as thus formed exhibits substantial photoconductive properties, said method comprising the steps of placing said previously formed photoconductive layer within a chamber which also contains a mass of said compound which is spaced from said photoconductive layer, and heating said mass and said photoconductive layer to a temperature between about 450 C. and 800 C.

2. In the method of claim 1, supplying said chamber with a halogen-containing atmosphere.

3. In the method of claim 2, causing said atmosphere to flow over said mass and said photoconductive layer.

4. The method of claim 2, in which said atmosphere also comprises oxygen.

5. The method of claim 2, in which said mass is in the form of a powder.

6. The method of claim 5, in which said atmosphere also comprises oxygen.

7. In the method of claim 5, causing said atmosphere to fiow over said mass and said photoconductive layer.

8. In the method of claim 1, supplying said chamber with an atmosphere comprising an inert gas and a member of the group consisting of halogen and hydrogen halide gases.

9. The method of claim 8, in which said atmosphere also comprises oxygen.

10'. In the method of claim 8, causing said atmosphere to flow over said mass and said photoconductive layer.

11. The method of claim 10, in which said mass is in the form of a powder.

12. The method of claim 1, in which said atmosphere also comprises oxygen.

13. The method of claim 1, in which said mass and said photoconductive layer are heated to a temperature between about 550 C. and 620 C.

14. In the method of claim 13, supplying said chamber with a halogen-containing atmosphere.

15. The method of claim 14, in which said mass is in the form of a powder.

16. In the method of claim 14, causing said atmosphere to flow over said mass and said photoconductive layer.

17. The method of claim 16, in which said mass is in the form of a powder.

18. In the method of claim 13, supplying said chamber with an atmosphere comprising an inert gas and a member of the group consisting of halogen and hydrogen halide gases.

19. In the method of claim 18, causing said atmosphere to flow over said mass and said photoconductive layer.

20. The method of claim 19, in which said mass is in the form of a powder.

References Cited UNITED STATES PATENTS 2,879,182 3/1959 Pakswer et al. 117 201 3,065,113 11/1962 Lyons 117 201 3,109,753 11/1963 Cole 117 201 3,145,120 8/1964 Cherolf et a1. 117-201 WILLIAM L. JARVIS, Primary Examiner. 

